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Paper I - General Linear Coordinates

When reading a FrameSet from a FitsChan, these conventions are used if the CTYPEi keyword values within the FitsChan do not conform to the conventions described in later papers, in which case the axes are assumed to be linear. When writing a FrameSet to a FitsChan, these conventions are used for axes which are described by a simple Frame (i.e. not a SkyFrame, SpecFrame, etc.).

Table [*] describes the use made by AST of each keyword defined by FITS-WCS paper I.


Table: Use of FITS-WCS Paper I keywords
Keyword Read Write
WCSAXESa Ignored. Set to the number of axes in the WCS Frame - only written if different to NAXIS.
CRVALia Used to create the pixel to WCS Mapping. Always written (see ``Choice of Reference Point'' below).
CRPIXja Used to create the pixel to WCS Mapping. Always written (see ``Choice of Reference Point'' below).
CDELTia Used to create the pixel to WCS Mapping. Only written if the CDMatrix attribute of the FitsChan is set to zero.
CROTAi Used to create the pixel to WCS Mapping. Only written in FITS-AIPS and FITS-AIPS++ encodings.
CTYPEia Used to choose the class and attributes of the WCS Frame, and to create the pixel to WCS Mapping (note, ``STOKES'' and ``COMPLEX'' axes are treated as unknown linear axes). Always written (see ``Use and Choice of CTYPE keywords'' below).
CUNITia Used to set the Units attributes of the WCS Frame. Only written if the Units attribute of the WCS Frame has been set explicitly. If so, the Units value for each axis is used as the CUNIT value.
PCi_ja Used to create the pixel to WCS Mapping. Only written if the CDMatrix attribute of the FitsChan is set to zero.
CDi_ja Used to create the pixel to WCS Mapping. Only written if the CDMatrix attribute of the FitsChan is set to a non-zero value.
PVi_ma Ignored for linear axes. Not written if the axes are linear.
PSi_ma Ignored. Not used.
WCSNAMEa Used to set the Domain attribute of the WCS Frame. Only written if the Domain attribute of the WCS Frame has been set explicitly. If so, the Domain value is used as the WCSNAME value.
CRDERia Ignored. Not used.
CSYERia Ignored. Not used.





Subsections

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AST A Library for Handling World Coordinate Systems in Astronomy
Starlink User Note 211
R.F. Warren-Smith & D.S. Berry
24th May 2011
E-mail:ussc@star.rl.ac.uk

Copyright (C) 2009 Science \& Technology Facilities Council